Ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and the like have been used as a solvent for an electrolyte solution of a lithium-ion secondary battery which is one of nonaqueous electrolyte secondary batteries. Since these organic solvents have excellent performance such as solubility of lithium compounds and high electrical conductivity, they are widely used as solvents for electrolyte solutions.
In recent years, in order to further improve efficiency of nonaqueous electrolyte secondary batteries, the performance improvement by adding a phosphonium salt to a nonaqueous electrolyte solution has been attempted (for example, refer to Patent Documents 1 to 4).
For example, Patent Document 1 proposes improving the low-temperature characteristics of electrochemical devices with a phosphonium salt compound using an anionic compound having an asymmetric structure as the anion portion of the phosphonium salt. However, when such a compound is used as an electrolyte solution of a lithium-ion secondary battery, there is also a problem of reduction in discharge capacity and cycle characteristics, and sufficient battery capacity is not obtained.
Patent Documents 2 to 4 propose using an electrolyte solution obtained by adding a phosphonium salt to an organic electrolyte solution for a lithium secondary battery. However, although these electrolyte solutions provide improved cycle characteristics, they had a problem of reduction in the initial discharge capacity.    Patent Document 1: Japanese Patent Laid-Open No. 2004-43407    Patent Document 2: Japanese Patent Laid-Open No. 9-63643    Patent Document 3: Japanese Patent Laid-Open No. 2004-87145    Patent Document 4: Japanese Patent Laid-Open No. 63-121268